The present invention relates to polyvinyl alcohol having sulfonic acid group-containing side chains, a solid polymer electrolyte membrane, a composite polymer membrane, a method for producing the same and an electrode.
In recent years, fuel cells have occupied an important position as next generation type clean energy sources. Of these fuel cells, a solid polymer electrolyte type fuel cell is one in which both anode and cathode electrodes are each arranged across a solid polymer electrolyte membrane intervening therebetween. For example, in the case of a direct methanol type fuel cell (hereinafter referred to as a xe2x80x9cDMFCxe2x80x9d) in which methanol is used as a fuel, methanol is supplied to the anode side, and oxygen or air to the cathode side, thereby allowing electrochemical reaction to occur to generate electricity. Solid polymer electrolyte membranes having high proton conductivity have been developed for retaining the characteristics of their high output and high energy density, and for obtaining small-sized, lightweight fuel cells. The solid polymer electrolyte membrane used in the DMFC is required to have the barrier property to fuel methanol, that is to say, reduced permeability (cross-over) of fuel methanol from the anode side of the membrane to the cathode side thereof.
Previously, hydrated membranes of perfluorosulfonic acid polymers such as Nafion (trade name) manufactured by E. I. du Pont de Nemours and Company) have generally been used as the solid polymer electrolyte membranes.
The above-mentioned hydrated membranes of perfluorosulfonic acid polymers have high proton conductivity, and the proton conductivity is exhibited by the generation of a channel structure caused by hydration (conduction of hydrated protons). That is to say, the conduction of protons takes place through water as a medium in the hydrated membranes of perfluorosulfonic acid polymers, so that a specified amount of water exists in the hydrated membranes. Accordingly, methanol having high affinity with water easily passes through the membranes, so that the hydrated membranes of perfluorosulfonic acid polymers have a limitation with regard to the methanol barrier property.
As means for reducing the cross-over of methanol through the hydrated membranes of perfluorosulfonic acid polymers, composites of different kinds of materials based on the perfluorosulfonic acid polymer membranes may be mentioned. However, the above-mentioned composites have significantly lower proton conductivity than the essential one of the hydrated membranes of perfluorosulfonic acid polymers.
The materials to be used with the above-mentioned perfluorosulfonic acid polymer membranes include polyvinyl alcohol (hereinafter also referred to as xe2x80x9cPVAxe2x80x9d) represented by the following general formula (I): 
Crosslinked PVA can be used as water/alcohol separation membranes in prevaporation [Ji-Won Rhimet et al., Journal of Applied Polymer Science, 68, 1717 (1998)].
However, PVA is significantly low in proton conductivity, so that composite polymer membranes obtained by using the perfluorosulfonic acid polymer membranes and PVA are substantially decreased in performance (proton conductivity) required for the solid polymer electrolyte membranes. As a consequence, the composite polymer membranes of the perfluorosulfonic acid polymer membranes and PVA have been difficult to be utilized as the solid polymer electrolyte membranes.
Further, as an electrode used in a solid polymer electrolyte type fuel cell, a so-called MEA (membrane electrode assembly) is known. In the MEA, electrodes are formed of fine catalyst particles prepared by allowing carbon to support a noble metal, a solid polymer electrolyte component formed on surfaces of the fine catalyst particles, and a fluorine resin for adhering the fine catalyst particles to one another. The electrodes are each arranged on two main planes of a solid polymer electrolyte membrane, thereby constituting a part of a fuel cell (Japanese Unexamined Patent Publication No. 5-36418). Then, the development of solid polymer electrolyte components formed on the surfaces of the fine catalyst particles for electrodes has been desired.
The present invention has been made against a background of the current problems of the solid polymer electrolyte materials as described above.
An object of the invention is to provide polyvinyl alcohol having sulfonic acid group-containing side chains.
Another object of the invention is to provide a solid polymer electrolyte.
Still another object of the invention is to provide a composite polymer membrane excellent in proton conductivity and methanol barrier property.
A further object of the invention is to provide a method for producing the same.
A still further object of the invention is to provide an electrode excellent in catalytic activity.
The present inventors have conducted intensive investigation for attaining the above-mentioned objects. As a result, the inventors have discovered that the characteristics of a solid polymer electrolyte having proton conductivity can be imparted to PVA by introducing side chains having sulfonic acid groups and crosslinked structures into PVA, and further that a composite polymer membrane excellent in proton conductivity and methanol barrier property is obtained by applying PVA, a sulfonating agent and a crosslinking agent solution to a water-absorptive or hydrophilic polymer membrane, followed by sulfonation and crosslinking to produce the composite polymer membrane, thus completing the invention.
The inventors have further discovered that polyvinyl alcohol having sulfonic acid group-containing side chains and crosslinked structures is suitable for a solid polymer electrolyte formed on the surfaces of fine catalyst particles carried on porous particles for an electrode, thus completing the invention.
The invention provides PVA having sulfonic acid group-containing side chains and crosslinked structures.
In the above-mentioned sulfonic acid group-containing polyvinyl alcohol, the sulfonic acid group-containing side chains and the crosslinked structures are preferably bonded to a main chain of polyvinyl alcohol by COO and/or OCO bonds.
The invention further provides a solid polymer electrolyte comprising the above-mentioned sulfonic acid group-containing polyvinyl alcohol.
It is preferred that the above-mentioned solid polymer electrolyte has water absorption.
Still further, the invention provides a composite polymer membrane in which the sulfonic acid group-containing polyvinyl alcohol described in any one of the above is formed on a surface of a water-absorptive or hydrophilic polymer membrane.
Yet still further, the invention provides a method for producing the composite polymer membrane described above, which comprises applying a solution of PVA, a sulfonating agent and a crosslinking agent to a water-absorptive or hydrophilic polymer membrane, followed by sulfonation and crosslinking.
Furthermore, the invention provides an electrode comprising the solid polymer electrolyte described above and fine catalyst particles carried on porous particles.